1. Field of the Invention
The present invention generally relates to determining critical area in integrated circuit designs and more particularly to an improved methodology of computing critical area for composite fault mechanisms.
2. Description of the Related Art
Critical area of a very large scale integration (VLSI) layout is a measure that reflects the sensitivity of the layout to defects occurring during the manufacturing process. Critical area is widely used to predict the yield of a VLSI chip. Yield prediction is essential in today's VLSI manufacturing due to the growing need to control cost. Models for yield estimation are based on the concept of critical area which represents the main computational problem in the analysis of yield loss due to random (spot) defects during fabrication. Spot defects are caused by particles such as dust and other contaminants in materials and equipment and are classified into two types: “extra material” defects causing shorts between different conducting regions and “missing material” defects causing open circuits.
In some defect modeling techniques, defects are modeled, consistently, as circles. The underlying reason for modeling defects as circles is the common use of Euclidean geometry. The distance between two points, usually, is measured by the length of the line segment joining the two points. This is the Euclidean distance. The locus of points a unit distance from a center point is usually called the “unit circle”. In Euclidean geometry, the “unit circle” is a circle of radius one.
In reality, spot defects are not necessarily circular. They can have any kind of shape. Therefore, it seems appropriate to use other geometries if the critical area computation can be simplified by modeling defects as squares, diamonds or octagons, respectively. For practical purposes, a circular defect can certainly be approximated by a regular octagon. Yield estimation should not considerably depend on which of the above geometries is used to model defects as long as the geometry is chosen consistently. Therefore, the geometry used for a particular computation, preferably, should allow critical area computation in the most efficient way.
A Voronoi diagram can also be used to enhance the computation of critical area. A Voronoi diagram of a set of 2D geometric elements (polygons, line segments, points) is a partition of the plane into regions representing those points on the plane closest to a particular geometric element. Here, “closest” is defined in terms of an appropriate geometry as mentioned above. These regions are called Voronoi cells, each of which is associated with its defining geometric element, called the owner of the cell. The set of points which separates two Voronoi cells is called a Voronoi bisector. The point where three or more Voronoi bisectors (or Voronoi cells) meet is called a Voronoi vertex.
Based on the circuit design and under an appropriate geometry, Voronoi diagrams can be constructed to model the effect of extra-material and missing-material spot defects. The Voronoi diagram partitions the circuit design into Voronoi cells within which defects that occur cause electrical faults between the same two shape edges in the design. This information can then be used to compute critical area. (e.g., see U.S. Pat. Nos. 6,317,859, 6,247,853, and 6,178,539, which are incorporated herein by reference).